Steam generator

ABSTRACT

A stream generator wherein a continuous evaporating heating surface is disposed in a heating gas duct through which a heating gas flows in an approximately horizontal manner. The continuous evaporating heating surface comprises a plurality of steam-generating pipes which are connected in a parallel manner enabling flowthrough of a flow medium, and is configured in such a manner that a steam-generating pipe which is heated more than another stream-generating pipe of the same continuous evaporating heating surface has a higher throughput of a flow medium compared to the other steam-generating pipe. An object is to produce a particularly low-cost steam generator exhibiting particularly high mechanical stability with different thermal loads. The steam-generating pipe respectively comprises a riser pipe piece which can be cross-flown in an upward direction and which is arranged in an approximately vertical manner in relation to the flow medium; a down pipe piece which can be cross-flown in a downward direction downstream from said flow medium side and another riser pipe piece which can be cross-flown in an upward direction by a flow medium and which is arranged in an approximately vertical manner downstream from the latter flow medium side. Preferably, the other riser pipe piece of the respective steam-generating pipe is arranged in the heating gas duct, when seen in the direction of the heating gas, between the riser pipe piece associated therewith and the down pipe piece associated therewith.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2003/013879, filed Dec. 8, 2003 and claims the benefitthereof. The International Application claims the benefits of Europeanapplication No. 03002243.8 EP filed Jan. 31, 2003, all of theapplications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a steam generator wherein a continuousevaporating heating surface is disposed in a heating gas duct throughwhich gas flows in an approximately horizontal manner, said continuousevaporating heating surface comprising a plurality of steam-generatingpipes which are connected in a parallel manner, enabling flowthrough ofthe flow medium and is configured in such a manner that a steamgenerating pipe which is heated more than another steam generating pipeof the same continuous evaporating heating surface has a higherthroughput of the flow medium compared to the other steam-generatingpipe.

BACKGROUND OF INVENTION

In a gas and steam turbine system the heat contained in the expandedoperating medium or heating gas from the gas turbine is used for thegeneration of steam for the steam turbine. The heat is transferred in anexhaust heat steam generator connected downstream from the gas turbine,in which a number of heating surfaces are usually arranged for water prewarming, for evaporation of the water and for steam superheating. Theheating surfaces are connected into the water-steam circulation of thesteam turbine. The water-steam circulation usually comprises a number ofpressure stages, for example three, with each pressure stage able tohave a continuous evaporating heating surface.

There are a number of alternative design concepts to be considered forthe steam generator connected downstream from the gas turbine on theheating gas side as a waste heat steam generator, namely a throughflowsteam generator design or a recirculating steam generator design. With athroughflow steam generator the heating of the steam generator pipesprovided as evaporation pipes leads to an evaporation of the flow mediumin the steam-generating pipes in a single pass. By contrast, with anatural or forced circulation steam generator, the water introduced intothe circulation is only partly evaporated when it passes through theevaporation pipes. The water not evaporated in this case is fed backagain, once it has been separated from the steam generated, to the sameevaporation pipes for a further evaporation.

By contrast with a natural or forced circulation steam generator athroughflow steam generator is not subject to any pressure limiting sothat fresh steam pressures far in excess of the critical pressure ofwater (P_(Kri)≈221 bar)—where only small differences in density betweenliquid-like and steam-like medium—are possible. A high fresh steampressure promotes a high degree of thermal efficiency and thereby lowCO₂ emissions of a fossil fuel-fired power station. In addition athroughflow steam generator has a simpler construction compared to arecirculating steam generator and can thus be manufactured atparticularly low cost. The use of a steam generator designed inaccordance with the flowthrough principle as a waste heat steamgenerator of a gas and steam turbine system is thus particularly usefulfor achieving a high overall efficiency of the gas and steam turbinesystem with a simple construction.

A waste heat steam generator constructed in a horizontal manner hasparticular advantages as regards the effort involved in manufacturingit, but also as regards the required maintenance work, the heatingmedium or heating gas, that is the waste gas from the gas turbine, beingfed in an approximately horizontal direction of flow through the steamgenerator. In a horizontal-design throughflow steam generator thesteam-generation pipes of a heating surface can however be subjected towidely different heating, depending on their positioning. Especiallywith steam generation pipes connected on the output side to a commoncollector, a different heating of individual steam generation pipes canresult in a merging of steam flows with sharply differing steamparameters and thereby to undesired losses of efficiency, especially toa comparatively reduced effectiveness of the heating surface involvedand thereby to reduced steam generation. A different heating of adjacentsteam-generating pipes can also, especially in the inlet area ofcollectors, lead to damage to the steam-generating pipes or to thecollector. The use of a horizontal design of throughflow steamgenerator, which is desirable per se, as a waste heat steam generatorfor a gas turbine, can thus present significant problems as regards asufficiently stabilized flow feed.

A steam generator is known from EP 0 944 801 B1 which is suitable for anarrangement as a horizontal design and also features the statedadvantages of a throughflow steam generator. To this end the known steamgenerator is designed as regards its continuous evaporating heatingsurface so that a steam-generatng pipe which is heated more incomparison with a further steam-generating pipe of the same continuousevaporating heating surface has a higher throughflow of the flow mediumcompared to the further steam-generating pipe. The continuousevaporating heating surface of the known steam-generator thus exhibits aself-stabilizing behavior in the type of flow characteristics of anatural recirculating continuous evaporating heating surface (naturalrecirculating characteristic) on occurrence of different heating ofindividual steam-generating pipes which without requiring any outsideinfluence, leads to a balancing out of the outlet-side temperatures evenat differently heated, flow-medium side steam-generating pipes connectedin parallel. However the known steam generator is comparativelyexpensive to construct, especially as regards the water and/or heat-sidedistribution of the flow medium.

SUMMARY OF INVENTION

An object of the invention is thus to specify a steam generator of thetype stated above which is particularly cheap to build and which alsoexhibits an especially high mechanical stability at different thermalloading.

This object is achieved accordance with the invention by one or each ofthe steam-generating pipes having an almost vertically arranged riserpipe piece through which the flow medium can flow in an upwardsdirection, a down pipe piece connected downstream on the flow mediumside, arranged almost vertically and through which the flow medium canflow in an downwards direction and a further riser pipe piece arrangedon the flow medium side downstream from the down pipe piece, throughwhich the flow medium can flow in an upwards direction.

In this case The invention uses as its starting point the considerationthat in a steam generator that can be constructed with especially lowinstallation and manufacturing outlay for an operating behavior which isespecially stable and especially insensitive in relation to differencesin the thermal stress, the design principle employed for the known steamgenerator, a natural recirculating characteristic for a continuousevaporating heating surface, is to be explicitly extended and furtherimproved. The steam generator continuous evaporating heating surfaceshould in this case be designed for application of a comparatively smallmass volume density with comparatively low frictional pressure loss.

An especially simple and thereby also robust construction can beachieved in this case by the heating surface being designed in aparticularly simple way especially as regards collection anddistribution of the flow medium. In this case the heating surface issuitably embodied for execution of all parts of the complete evaporationprocess, that is of pre-warming, evaporation and at least partsuperheating in just one single stage, that is without intermediatecomponents to collect and/or distribute the flow medium. There isgenerally provision for additional heating surfaces for pre-warming ofthe feed water or for further superheating. In order in this case to onthe one hand be able to undertake all of the stated parts of the processcompletely in the relevant steam-generating pipe and on the other handto allow sufficient flexibility in the adaptation of thesteam-generating pipes to the requirements of these process parts andthe method in the heating gas duct, each steam-generating pipe comprisesthree segments connected one after the other on the flow medium side.

In order to also support the desired natural recirculationcharacteristic of the through flow in this design a division of thesteam-generating pipes of the continuous evaporator heating surface intoat least three segments (of parallel pipes) is provided, with the firstsegment comprising all riser pipe pieces through which the flow is in anupwards direction. Correspondingly the second segment comprises all downpipe pieces and the medium flows through these in a downwards directionso that the flow is supported automatically by the inherent weight ofthe flow medium. In this case the down pipe pieces forming the secondsegment of each steam-generating pipe in the heating and gas duct arearranged, viewed in the direction of the heating gas, behind the riserpipe pieces assigned to them in each case. The third segment comprisesall further riser pipe pieces and the flow material flows upwardsthrough it.

In an especially advantageous embodiment the segments of thesteam-generating pipe or of each steam-generating pipe in the heatinggas duct are positioned such that the heating requirement of eachsegment—especially with respect to the stage in the evaporation processprovided there—is adapted in a particular way to the local heat providedin the heating gas duct. To this end the further riser or pipe piecesforming the third segment of each steam-generating pipe are expedientlyarranged in the heating gas duct, viewed in the heating gas direction,between the riser pipe pieces of the first and the down pipe pieces ofthe second segment assigned to them. In other words: Thesteam-generating pipes are expediently positioned spatially in theheating gas duct so that the first segment viewed from the flowmedium-side is arranged upstream from the third segment or further riserpipe piece viewed from the flow medium side and the second segment ordown pipe piece viewed from the flow medium-side is arranged upstream onthe heating gas side from the third segment or further riser pipe pieceviewed from the flow medium side.

In this type of arrangement the first riser pipe piece which is used forpartial preheating and already to a large extent for evaporation of theflow medium is thus subjected to a comparatively strong heating by theheating gas in the hot smoke gas area. This ensures that in the overallrange of loads, flow medium flows out of the relevant first riser pipepiece with a comparatively high steam component. This leads, onsubsequent introduction into the downstream down pipe piece, to anincrease in a steam bubbles against the flow direction of the flowmedium which would be bad for the flow stability in the down pipe piecebeing explicitly avoided. Arranging the down pipes piece in thecomparatively cold smoke gas area and arranging the second riser pipepiece between the first riser pipe piece and the down pipe piece, thatis on the smoke gas side before the down pipe piece, thus achieves anespecially high degree of efficiency with higher operational safety, inwhich case the first riser pipe piece fulfills the function of apre-evaporator.

An especially simple design of the continuous evaporating heat surfaceon the one hand and an especially low mechanical stress of thecontinuous evaporating heat surface, even with different thermal loads,on the other hand can be achieved in that in a further or alternativelyadvantageous design the riser pipe piece of one or all of eachsteam-generating pipe with the down pipe piece assigned to it as well asthe down pipe piece of one or all of each steam-generating pipe with thefurther riser pipe piece assigned to it is connected on the flowmedium-side via a cross flow piece in each case.

This type of arrangement is especially suitable for expansioncompensation with alternating thermal loading in that the riser pipepiece and the down pipe piece or the cross flow piece connecting thedown pipe piece and the further riser pipe piece serve in this casenamely as expansion bends which can compensate for the relative changesin length of the riser pipe piece and/or the down pipe piece and/or thefurther riser pipe piece in any event. The cross flow piece thusprovides a diversion of the steam-generating pipes in the upper area ofa first evaporator stage produced by the riser pipe pieces with directforwarding and new diversion in the lower area of a second evaporatorstage formed by the down pipe pieces as well as a diversion andforwarding of the steam-generating pipes in the lower area of the secondevaporating stage into a third evaporator stage formed by the furtherriser pipe pieces.

The cross flow piece or each cross flow piece is advantageously laidwithin the heating gas duct. Alternatively the cross flow piece canhowever also be routed outside the heating gas duct, especially if,because a possible draining of the continuous evaporation heatingsurface is required, a drain water collector is to be connected to thecross flow piece.

The steam-generating pipes can be grouped together within the heatinggas to form rows of pipes of which each row features a number ofsteam-generating pipes arranged alongside one another at right angles tothe heating gas direction. With this type of embodiment thesteam-generating pipes are advantageously routed such that the riserpipe pieces forming the row of pipes heated most strongly, that is thefirst row of pipes viewed in the heating gas direction, is assigned tothe row of down pipe pieces heated the least of the last row of pipesviewed in the heating gas direction. In addition the down pipe and riserpipe pieces of a number of steam-generating pipes are expedientlypositioned in the heating gas duct relative to one another such that adown pipe piece lying relatively far back when viewed in the heating gasdirection is assigned to a further riser pipe piece lying comparativelyfar forward viewed in a heating gas direction.

Through this type of arrangement the comparatively strongly heatedfurther riser pipe pieces are fed with a comparatively weakly preheatedflow medium flowing out of the down pipe pieces.

To ensure the natural recirculation characteristic desired for a sterilethroughflow of the pipes, the relevant steam-generating pipe isadvantageously embodied such that it merely comprises a riser pipe pieceas well as a down pipe piece connected to it downstream on the flowmedium side as well as a further riser pipe piece connected downstreamof the latter on the flow medium side.

Expediently the steam generator is used as a waste heat steam generatorof a gas and steam turbine system. In this case the steam generator isadvantageously connected downstream from a gas turbine on the heatinggas side. In this circuit an additional firing unit can expediently bearranged behind the gas turbine to increase the heating gas temperature.

The advantages obtained with the invention consist in particular in thefact that with a three-stage embodiment of the steam-generating pipeswith a riser pipe piece through which the medium can flow in an upwardsdirection, a down pipe piece through which the medium can flow in adownwards direction and a further riser pipe piece connected downstreamfrom this on the flow medium side through which the medium can flow inan upwards direction, the complete execution of the evaporation, that ispart preheating, evaporation and part superheating can be achieved injust one stage and without intermediate connection of components forcollection or distribution in an especially simple design. In this casefor example a layout without water separator is possible, with anundesired water surge in the superheater being able to be avoided orkept low on startup, in that at the beginning of the startup processexclusively the relevant first riser pipe piece is filled with water,that after the beginning of the startup process on passage through thesubsequent pipe pieces evaporation takes place completely or to asufficiently high degree.

Heated evaporator systems with an upwards flow do usually lead to flowinstabilities which are simply not tolerable for use in forcethroughflow evaporators. For a throughflow with comparatively low massdensity flows the comparatively low friction pressure loss means that anatural recirculation characteristic of the steam generator can beachieved in a reliable manner, which leads on multiple heating of asteam-generating pipe compared to a further steam-generating pipe to acomparatively higher throughflow of the flow medium in the multiplyheated steam-generating pipe. This natural circulation characteristicguarantees, even when upwards-flowed pipe pieces are used, asufficiently stable and reliable throughflow of the steam-generatingpipes.

Such a characteristic can also be achieved with especially lowconstructional and installation outlay, in that the down pipe piece isconnected directly downstream of the riser pipe piece assigned to it ineach case or the further riser pipe piece is directly connecteddownstream from the down pipe piece assigned to it in each case andwithout intermediate connection of an expensive collection ordistribution system. The steam generator thus exhibits a comparativelylow system complexity with especially stable flow behavior. In additionboth the riser pipe piece and also the down pipe piece and the furtherriser pipe piece connected downstream from this of each steam-generatingpipe can be attached as a hanging unit in the area of the housing coverof the heating gas duct, with a free lengthwise expansion being allowedin the lower area. These types of lengthwise expansion resulting fromthermal effects are now compensated for by the cross flow piececonnecting the relevant down pipe piece with the riser pipe piece or thefurther riser pipe piece with the down pipe piece so that no strainsarise as a result of thermal effects.

BRIEF DESCRIPTION OF THE DRAWING

Exemplary embodiments of the invention are explained in greater detailwith reference to a drawing. In this drawing the FIGURE shows asimplified diagram of a lengthwise cross piece or a steam generator in ahorizontal design.

DETAILED DESCRIPTION OF INVENTION

The steam generator 1 in accordance with the FIGURE is connecteddownstream as a type of waste heat steam generator from a gas turbine 40not shown in any greater detail. The steam generator 1 features asurrounding wall 2 which forms a heating gas duct 6 through which flowis possible in an almost horizontal heating gas direction x, indicatedby the arrows 4 for the exhaust gas from the gas turbine. In the heatinggas duct 6 are a number of heating surfaces each arranged in accordancewith the throughflow principle, also designated as the continuousevaporation heating surface 8, which are provided for the evaporation ofthe flow medium. Only one continuous evaporating heating surface 8 isshown in the exemplary embodiment in accordance with the FIGURE, but alarger number of continuous evaporating heating surfaces can beprovided.

The evaporation system formed from the continuous evaporating heatingsurface 8 can have a flow medium W applied to it which evaporates on asingle pass through the continuous evaporating heating surface 8 andafter leaving the continuous evaporating heating surface 8 as alreadysuperheated steam D is discharged and merely fed if required for furthersuperheating to superheater heating surfaces. The evaporator systemformed from the continuous evaporating heating surface 8 is connectedinto the water-steam circulation of a gas turbine which is not shown inany greater detail. In addition to the evaporator system a number offurther heating surfaces 10 shown schematically in FIG. 1 are connectedinto the water-steam circulation of the steam turbine. The heatingsurfaces 10 can for example be superheaters, medium-pressureevaporators, low-pressure evaporators and/or preheaters.

The continuous evaporating heating surface 8 of the steam-generator 1 inaccordance with the FIGURE comprises a plurality of steam-generatingpipes 12 in the form of a pipe bundle connected in parallel forthroughflow by the flow medium W. In this case a plurality ofsteam-generating pipes 12 viewed in the heating gas direction x, arearranged next to each other. In this case only one of thesteam-generating pipes 12 arranged next to one another in this way isvisible. A common distributor 16 is connected upstream from thesteam-generating pipes 12 arranged next to each other in this way on theflow medium side in each case and a common outlet collector 18 isconnected downstream in each case. The distributors 16 are in this casefor their part connected on their input side to a main distributor 20,with the outlet collectors 18 being connected on their output side to amain collector 22.

The continuous evaporating heating surface 8 is designed so that it issuitable for feeding the steam-generating pipes 12 with comparativelylow mass flow densities, with the steam-generating pipes 12 exhibiting anatural circulation characteristic. With the natural circulationcharacteristic a steam-generating pipe 12 heated more in comparison witha further steam-generating pipe 12 of the same continuous evaporatingheating surface 8 has higher throughflow of the flow medium W incomparison to the further steam-generating pipe 12. To ensure this withespecially simple constructive means in an especially reliable way, thecontinuous evaporating heating surface 8 features three segmentsconnected in series on the flow medium side. In the first segment eachsteam-generating pipe 12 of the continuous evaporating heating surface 8features an almost vertically-arranged riser pipe piece 24 through whichthe flow medium W can flow in an upwards direction. In the secondsegment each steam-generating pipe 12 features an almost verticallyarranged down pipe piece 26 connected downstream from the riser pipepiece 24 on the flow medium side and through which the flow medium W canflow in a downwards direction. In the third segment eachsteam-generating pipe 12 features a further riser pipe piece 28 arrangedalmost vertically and connected downstream from the down pipe piece 26on the flow medium side and through which the flow medium W can flow inan upwards direction.

Viewed in the heating gas direction x the segment formed by the furtherriser pipe piece 28 is arranged between the segment formed by the firstriser pipe pieces 24 and the segment formed by the down pipe pieces 26.This ensures a construction which is matched to a particular degree tothe requirements for the heating of the flow medium and to the heatingcircumstances in the heating gas duct 6.

The down pipe piece 26 is connected to the riser pipe piece 24 assignedto it in this case via a cross flow piece 30. In the same way thefurther riser pipe piece 28 is connected to the down pipe piece 26assigned to it via a cross flow piece 30. In the exemplary embodimentthe cross flow pieces 30 are routed within the heating gas duct 6.Alternatively the cross flow pieces 30 can also be routed outside theheating gas duct 6. This can be especially useful for the case in which,for constructional or operational reasons, draining of the continuousevaporating heating surface 8 is to be provided.

As can be seen from the FIGURE, a down pipe piece 26 with the furtherriser pipe piece 28 assigned to it and the cross flow piece 30connecting the two is shaped almost like a U, with the uprights of the Ubeing formed by the down pipe piece 26 and the further riser pipe piece28 and the connecting bend being formed by the cross flow piece 30. Witha steam-generating pipe 12 designed in this way the geodetic pressurecontribution of the flow medium W in the area of the down pipe piece26—by contrast with the area of the further riser pipe piece 28—createsa flow-promoting and not a flow-inhibiting pressure contribution. Inother words: The water column of unevaporated flow medium W located inthe down pipe piece 26 “pushes” the throughflow of the relevantsteam-generating pipe 12 with it instead of inhibiting it.

This means that the steam-generating pipe 12, viewed overall, exhibits acomparatively low pressure loss.

With this type of construction the two riser pipe pieces 24, 28 and thedown pipe piece 26 are hung or fixed onto the cover of the heating gasduct 6 in a kind of hanging construction 44. Viewed spatially, the lowerend of the relevant riser pipe piece 24 and the lower end of therelevant down pipe piece 26 and of the further riser pipe piece 28,which are each interconnected by a cross flow piece 30 are by contrastnot directly spatially fixed in the heating gas duct 6. Lengthwiseexpansions of these segments of the steam-generating pipes 12 can thusbe tolerated without damage, with the relevant cross flow piece 30operating as an expansion curve. This arrangement of thesteam-generating pipes 12 is thus mechanically especially flexible andinsensitive as regards thermal stresses in relation to differenceexpansions which occur.

Greater heating of a steam-generating pipe 12, especially in its riserpipe piece 24, leads in this case initially to an increase in theevaporation rate, with, because of the dimensioning of thesteam-generating pipe 12 as a consequence of this greater heating, anincrease in the throughflow rate through the more heatedsteam-generating pipe 12 occurs.

In addition the down pipe pieces 26 and the further riser pipe pieces 28of a number of steam-generating pipes 12 are positioned in the heatinggas duct 6 relative to one another such that riser pipe pieces 24, 28lying comparatively far forward viewed in the heating gas direction xare assigned to a down pipe piece 26 lying comparatively far back viewedin the heating gas direction x in each case. Through this arrangementcomparatively strongly heated riser pipe pieces 24, 28 communicate witha comparatively weakly heated down pipe piece 26. Through this relativepositioning an automatic balancing effect is also achieved between thepipe rows 14 in relation to the throughflow.

As a result of the particularly marked natural circulationcharacteristic of the steam-generating pipes 12, these exhibit to aparticular extent a self-stabilizing behavior in relation to locallydifferent heating: Stronger heating of a row of steam-generating pipes12 in this case leads locally to an increased feeding of flow medium Winto this row of steam-generating pipes 12, so that as a result of thecorresponding increased cooling effect an automatic equalization of therelevant temperature values is initiated. The fresh steam flowing intothe main collector 22 is thus especially homogeneous as regards itssteam parameters, regardless of the individual pipe row 14 through whichit has passed.

A particular advantage of the construction of the continuous evaporatingheating surface 8 of which the outlet in the form of the further riserpipe pieces 28 on the gas side between the first riser pipe pieces 24 onthe one hand and the down pipe pieces 26 on the other hand and isthereby positioned in a mid gas temperature area of the continuousevaporating heating surface 8, lies in the fact that through thispositioning too strong a superrheating of the flow medium even inindividual steam-generating pipes 12 is avoided in a natural way at theoutlet of the continuous evaporating heating surface 8.

1. A steam generator, comprising: a heating gas duct through which afirst medium can flow in a substantially horizontal heating direction;and a plurality of steam-generating pipes connected for a through flowof a second medium and arranged in the heating gas duct, eachsteam-generating pipe comprising: a first riser section through whichthe second medium can flow in an upward direction, the first risersection arranged substantially vertically, a down section in fluidcommunication to receive the second medium from the first riser sectionand through which the second medium can flow in a downwards direction,the down section arranged substantially vertically, and a second risersection in fluid communication to receive the second medium from thedown section and through which the second medium can flow in an upwarddirection, the second riser section arranged substantially vertically;wherein the second riser section is disposed within the heating gas ductbetween the first riser section and the down section relative to adirection of flow of the first medium, and the first riser section isdisposed within the heating gas duct upstream of the second risersection relative to the direction of flow of the first medium.
 2. Thesteam generator in accordance with claim 1, wherein eachsteam-generating pipe further comprises: a first cross flow piece thatconnects the first riser section to the down section, and a second crossflow piece that connects the down section to the second riser section.3. The steam generator in accordance with claim 2, wherein the first andsecond cross flow pieces are arranged outside the heating gas duct. 4.The steam generator in accordance with claim 2, wherein eachsteam-generating pipe is supported with a hanging construction nitwithin the heating gas duct with a free lengthwise expansion beingallowed in each of the first riser, down and second riser sections withthe first and second cross flow pieces operating as respective expansioncurves for accommodating thermal expansion.
 5. The steam generator inaccordance with claim 1, wherein a gas turbine is connected to the steamgenerator upstream on a gas side.
 6. The steam generator in accordancewith claim 1, wherein the plurality of steam-generating pipes comprise afirst pipe and a second pipe disposed generally parallel to each otherwithin the heating gas duct, each of the first pipe and the second pipecomprising respective first riser, down and second riser sections, andfurther comprising: the first pipe being disposed upstream of the secondpipe relative to the direction of flow of the first medium along theirrespective first and second riser sections; and the first pipe beingdisposed downstream of the second pipe relative to the direction of flowof the first medium along their respective down section.